Improved voice quality created by voice processing DSP (Digital Signal Processing) algorithms has been used to differentiate network providers. The transfer to packet networks or networks with extended tandem free operation (TFO) or transcoder free operation (TrFO) will diminish this ability to differentiate networks with traditional voice processing algorithms. Therefore, operators which have generally been responsible for maintaining speech quality for their customers are asking for voice processing algorithms to be carried out also for coded speech.
TFO is a voice standard to be deployed in the GSM (Global System for Mobile communications) and GSM-evolved 3G (Third Generation) networks. It is intended to avoid the traditional double speech encoding/decoding in mobile-to-mobile call configurations. The key inconvenience of a tandem configuration is the speech quality degradation introduced by the double transcoding. According to the ETSI listening tests, this degradation is usually more noticeable when the speech codecs are operating at low rates. Also, higher background noise level increases the degradation.
When the originating and terminating connections are using the same speech codec it is possible to transmit transparently the speech frames received from the originating MS (Mobile Station) to the terminating MS without activating the transcoding functions in the originating and terminating networks.
The key advantages of Tandem Free Operation are improvement in speech quality by avoiding the double transcoding in the network, possible savings on the inter-PLMN (Public Land Mobile Network) transmission links, which are carrying compressed speech compatible with a 16 kbit/s or 8 kbit/s sub-multiplexing scheme, including packet switched transmission, possible savings in processing power in the network equipment since the transcoding functions in the Transcoder Units are bypassed, and possible reduction in the end-to-end transmission delay.
In TFO call configuration a transcoder device is physically present in the signal path, but the transcoding functions are bypassed. The transcoding device may perform control and protocol conversion functions. In Transcoder Free Operation (TrFO), on the other hand, no transcoder device is physically present and hence no control or conversion or other functions associated with it are activated.
The level of speech is an important factor affecting the perceived quality of speech. Typically in the network side there are used automatic level control algorithms, which adjust the speech level to a certain desired target level by increasing the level of faint speech and somewhat decreasing the level of very loud voices.
These methods cannot be utilized as such in future packet networks where the speech travels in the coded format end-to-end from the transmitting device to the receiving device.
Currently the coded speech is decoded in the network and speech enhancement is carried out with linear PCM samples using traditional speech enhancement methods. After that the speech is encoded again, and transmitted to the receiving party.
However, for example, for AMR speech codec the level control is more difficult in the lower modes due to the fact that the fixed codebook gain is no longer scalar quantized but it is vector-quantized together with the adaptive codebook gain.